This application is based upon and claims benefit of priority of Japanese Patent Application No. 2000-328971 filed on Oct. 27, 2000, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a pressure sensor formed on a semiconductor substrate together with a circuit for processing signals from the pressure sensor.
2. Description of Related Art
Generally, in a semiconductor pressure sensor, it is necessary to calibrate its output errors caused by a manufacturing process fluctuation or other factors. The calibration is performed in a signal processor circuit connected to a detector element. An example of the output signal calibration is disclosed in an article entitled xe2x80x9cDSP-BASED CMOS MONOLITHIC PRESSURE SENSOR FOR HIGH VOLUME MANUFACTURINGxe2x80x9d on page 362-365 of xe2x80x9cTransducers ""99, Jun. 7-10, 1999, Sendai, Japan.xe2x80x9d This article teaches digital calibration of sensor outputs and storage of an amount of calibration in a non-volatile on-chip memory.
It is known to use CMOS elements in a signal processor circuit for a semiconductor pressure sensor, as disclosed in JP-A-8-64693 and JP-A-7-326771. The CMOS elements are usually formed on a silicon substrate having an (100)-surface-orientation, which has a low boundary energy level (hereinafter referred to as an (100)-type substrate), as taught in a book xe2x80x9cSemiconductor Devicexe2x80x94Basic Theory and Process Technologyxe2x80x9d (first published in 1987 and issued as the sixth edition in 1992, on page 205). It is generally known to make a semiconductor sensor compact by forming the signal processor circuit that includes CMOS elements on a chip together with the sensor element.
On the other hand, JP-A-4-119672 suggests that the pressure sensor formed on the (100)-type substrate has a problem in accuracy, because an offset voltage is generated by thermal stress on a diaphragm and temperature dependency of the offset voltage is non-linear. It also suggests the offset voltage due to thermal stress may be reduced by using a silicon semiconductor substrate having an (110)-surface-orientation in making a discrete type pressure sensor.
However, there still remains a problem of output errors in a semiconductor pressure sensor, notwithstanding various suggestions made in prior art.
The present invention has been made in view of the above-mentioned problem, and an object of the present invention is to provide an improved semiconductor pressure sensor having high detection accuracy by minimizing an adverse influence of the thermal stress on the sensor output voltage.
The semiconductor pressure sensor according to the present invention is composed of a pressure detector element and a signal processor circuit, both formed on a semiconductor substrate having an (110)-surface-orientation (referred to as an (110)-type substrate). The pressure detector element includes a diaphragm and four strain gauges formed on the diaphragm. The diaphragm is distorted in response to a pressure imposed thereon, and the diaphragm distortion is converted into an electric signal by the strain gauges. The strain gauges formed as a diffusion layer on the substrate are connected to form a Wheatstone bridge. A constant voltage is supplied to the bridge as an input voltage, and the bridge outputs a signal voltage representing the pressure to be measured.
The signal processor circuit for processing the output voltage from the detector element includes CMOS elements or Bi-CMOS elements. The signal processor circuit has an amplifier for amplifying the output voltage from the detector element. The output voltage is inputted to the amplifier from its input circuit composed of a pair of transistors. The pair of transistors are positioned on the substrate so that current between a source and a drain in both transistors flows in the same direction.
Preferably, the strain gauges are positioned on the (110)-type substrate along its  less than 110 greater than  crystal axis, and the pair of transistors in the amplifier input circuit are positioned so that the source-drain current flows in the direction of the  less than 110 greater than  crystal axis. The diaphragm is formed in an octagonal shape to equalize thermal stress influence on each strain gauge. The signal processor circuit includes a non-volatile memory in which data for adjusting the detector output voltage are written from outside.
Since the pressure detector element and the signal processor circuit are formed on the (110)-type substrate, the influence of thermal stress on the sensor outputs is reduced, and thereby the pressure is detected with high accuracy. Since the pair of transistors in the amplifier input circuit are positioned on the substrate to equalize the source-drain current directions, the amplifier is able to amplify a low level detector output voltage with high accuracy. Temperature dependency of the sensor output voltage, which possibly exists at a low level, is easily adjusted based on the data in the non-volatile memory.
Other objects and features of the present invention will become more readily apparent from a better understanding of the preferred embodiment described below with reference to the following drawings.